Discharges
of oil from shipping, offshore extraction of oil, and transport
of oil in pipelines is the result of either accidents or "normal",
deliberate operational discharges. Accidental discharges
(oil spills) occur when vessels collide or come in distress
at sea (engine breakdown, fire, explosion) and break open, or
run aground close to the shore, or when there is a blowout of
an offshore oil well, or when a pipeline breaks. Much can be
done to avoid accidents, but there will always be unfortunate
circumstances and situations that cause accidents to happen.Operational discharges,
on the other hand, are mostly deliberate and "routine",
and can to a very large extent be effectively controlled and
avoided. It is much a question combining available technical
solutions with information, education and a change of attitude
among ship-owners, mariners, offshore platform and pipeline
operators. SPILLS
FROM SHIPS

Accidents involving oil tankers or offshore platforms or
oil pipelines have caused many and sometimes very large
oil spills. Such spills are the most obvious, visible and
dramatic causes of acute oil pollution of the marine environment.
However, the largest oil spill ever was caused by Iraq deliberately
released about 240 million gallons (about 800,000 tonnes)
of crude oil into the Persian Gulf during the 1991 Gulf
War and burnt oil wells in Kuwait.

According to estimates made of the contribution of oil to
the sea from different sources (NRC), accidents
involving oil tankers and offshore installations account for
some 10 per cent of the annual total amount of oils entering
the marine environment. As reported by INTERTANKO,
"tankers carry close to 40 per cent of the world's seaborne
trade. In 2001, 57 per cent of the oil consumed in the world
was transported by sea, approx. 2,000 million tonnes.

As pointed out by U.S. NOAA, "if
oil spills of all sizes are considered, tankers and offshore
installations do not account for most of these spills. Tanker
accidents and accidents with offshore installations do, however,
account for most of the world's largest oil spills. Accidental
spills are less frequent than other kinds of oil spills, but
typically involve large volumes of spilled oil relative to
other kinds of oil spills".

"Analysts for the Oil Spill Intelligence
Report track oil spills of at least 10,000 gallons (34
tonnes). In their annual International Oil Spill Statistics
report for 1999, they reported that in that year about 32
million gallons of oil spilled into the water or onto land,
in 257 incidents. Of those incidents, only 11 were spills
from tankers, accounting for about 6.6 million gallons, or
about one-fifth of the total volume of oil spilled. Twenty-five
of the 257 spills were from barges and other kinds of vessels,
such as freighters (totaling 1.5 million gallons). Eighteen
spills were from trucks or railroad trains (totaling about
half a million gallons). The largest number of spills, and
the largest volume of oil spilled were from accidents involving
pipelines or fixed facilities (131 pipeline spills, totaling
about 18.8 million gallons; 66 spills from facilities, totaling
about 4.7 million gallons). The percentages of oil spilled
from different sources vary greatly from year to year; in
some years, tanker accidents represent the largest single
source of spilled oil, but only in a very few years is it
the case that most of the oil spilled (in significant spills)
during that year came from tankers. However, tanker accidents
have been the cause of most of the very largest oil spills.
The Cutter Information Corporation analysts also have found
that of the 66 spills in which at least 10 million gallons
(34,000 tonnes) of oil were lost, 48 were from tankers. Eight
were from fixed facilities, especially storage tanks, five
were from production oil wells, three were from pipelines,
and two were from other kinds of cargo vessels."

"Oil spills happen all around the world. According to
the Oil Spill Intelligence Report, spills in the size range
of at least 34 tonnes have occurred in the waters of 112 nations
since 1960. However, oil spills happen more frequently in
certain parts of the world. They identified the following
'hot spots' for oil spills from vessels: the Gulf of Mexico
(267 spills); the northeastern U.S. (140 spills); the Mediterranean
Sea (127 spills); the Persian Gulf (108 spills); the North
Sea (75 spills); Japan (60 spills); the Baltic Sea (52 spills);
the United Kingdom and English Channel (49 spills); Malaysia
and Singapore (39 spills); the west coast of France and north
and west coasts of Spain (33 spills); and Korea (32 spills)".

However, one should not merely compare figures  the
size of the spill is certainly not the only factor of importance
in terms of what environmental
damage can be caused by a spill. One example only, from
the waters off South Africa: "There is rather little
correlation between the tonnages of oil released in spills
and the impacts on the marine ecosystems. For example, a collision
between two oil tankers in 1977 released 31,000 tonnes of
oil and polluted 47 African Penguins, but in the Apollo
Sea sinking of 1994, about 2,000 tonnes of oil impacted
about 10,000 penguins. After the Apollo Sea, we generally
believed that this was the maximum amount of penguin mischief
that 2,000 tonnes of oil could achieve. However, when the
Treasure sank on 23 June 2000, half this amount of
oil threatened four times as many penguins! In round figures,
20,000 penguins were oiled, and 20,000 penguins were prevented
from becoming oiled by removing them off their breeding colonies
on Dassen and Robben Islands."

Leakages from sunken, grounded or abandoned ships is
another potential (and often very real) source of oil to the
marine environment. These can be merchant or military vessels.
Sunken vessels means just that  ships that have sunk
to the bottom of the sea, generally due to an accident but
sometimes also as the result of a deliberate action to get
rid of them. Regarding grounded and abandoned vessels, a distinction
is made in the NRC report between derelict vessels and historic
wrecks. "Derelict vessels are generally unseaworthy or
are no longer useful and have been tied up and abandoned.
Others are mothballed, are awaiting repair or dismantling,
or are intentionally grounded as a result of illegal activity.
--- These abandoned vessels become potential sources of oil
pollution, from either chronic leaks or a large release once
oil storage areas fail." And: "From an oil pollution
perspective, wrecks sunken during and since World War II pose
the greatest risk because of the presence of residual fuels.
-- World War II wrecks are of particular concern because they
can contain large volumes of oil, and corrosion after nearly
60 years underwater can lead to chronic leaks and the potential
for catastrophic releases.

SPILLS
FROM OFFSHORE DRILLING

As described by APPEA, "the weight
of the drilling fluid/drilling mud acts as the first line
of well control by keeping underground pressures in check.
If an influx of pressurised oil or gas does occur during drilling,
well control is maintained through the rig's blowout prevention
system (BOP). This is a set of hydraulically operated valves
and other closure devices (rams) which seal off the well,
and route the wellbore fluids to specialised pressure controlling
equipment. Trained personnel operating this highly reliable
equipment minimise the possibility of a blowout, or an uncontrolled
flow of fluids from a well".

However, it does not always work. As pointed out in the book
Environmental Impact of the Offshore Oil and Gas Industry,
the most typical causes of blowouts "include
equipment failure, personnel mistakes, and extreme natural
impacts (seismic activity, ice fields, hurricanes, and so
on). Their main hazard is connected with the spills and blowouts
of oil, gas, and numerous other chemical substances and compounds.
The environmental consequences of accidental episodes are
especially severe, sometimes dramatic, when they happen near
the shore, in shallow waters, or in areas with slow water
circulation. Broadly speaking, two major categories of drilling
accidents should be distinguished. One of them covers catastrophic
situations involving intense and prolonged hydrocarbon gushing.
These occur when the pressure in the drilling zone is so high
that usual technological methods of well muffling do not help.
Lean holes have to be drilled to stop the blowout. Drilling
accidents are usually associated with unexpected blowouts
of liquid and gaseous hydrocarbons from the well as a result
of encountering zones with abnormally high pressure. No other
situations but tanker oil spills can compete with drilling
accidents in frequency and severity."

The largest blowout, so far, was the Ixtoc
I accident off the Mexican coast in 1979-1980 (about nine
months). With a total oil spill of about 475,000 tonnes, it
was the second largest oil spills of all times. There have
been other large blowouts since then, among them one in Nigeria
in 1980 with a total spill of over 54,000 tonnes (destroying
over 340 hectares of mangroves).

SPILLS
FROM PIPELINES

"The causes of underwater pipeline damage
can differ greatly. They range from material defects and pipe
corrosion to ground erosion, tectonic movements on the bottom,
and encountering ship anchors and bottom trawls. --- Depending
on the cause and nature of the damage (cracks, ruptures, and
others), a pipeline can become either a source of small and
long-term leakage or an abrupt (even explosive) blowout of
hydrocarbons near the bottom. The dissolution, dilution, and
transferring of the liquid and gaseous products in the marine
environment can be accompanied in some cases by ice and gas
hydrates formation. --- Modern technology of pipeline construction
and exploitation under different natural conditions, including
the extreme ones, achieved indisputable successes. However,
pipeline oil and gas transportation does not eliminate the
possibility of serious accidents and consequences."
(Environmental Impact of the Offshore Oil and Gas Industry)

Thus, pipelines can continuously leak oil for a long time,
which is bad enough, but when a pipeline ruptures it causes
a large, acute spill. The best-known examples of large spills
occurred in the Guanabara Bay, off the Brazilian coast in
2000, when about 1,300 tonnes of oil were released into the
sea, and in Nigeria in 1998, when a pipeline broke and 14,300
tonnes of oil were spilt.

SHIPS

U.S.
NOAA: Oil
Spill Case Histories Database, 1967-1991. Descriptions
of oil spills around the world from tankers, offshore oil
wells and pipelines. U.S. National Oceanic and Atmospheric
Administration (NOAA) Office of Response and Restoration.

U.S.
NOAA: Oil
Spill Case Histories Database, 1967-1991. Descriptions
of oil spills around the world from tankers, offshore oil
wells and pipelines. U.S. National Oceanic and Atmospheric
Administration (NOAA) Office of Response and Restoration.